Replaceable cp anodes

ABSTRACT

An anode for cathodic protection of underwater equipment is provided. The anode comprises: a support body; sacrificial material retained by the support body; and an attachment mechanism for releasably attaching the anode to the underwater equipment.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of invention relates generally to anodes for cathodicprotection of underwater-located equipment and a method for providingcorrosion protection of underwater-located equipment. More specifically,the field of invention relates to cathodic protection of well trees forsubsea hydrocarbon fluid extraction.

2. Description of Related Art

Metallic equipment that is to be deployed underwater is at risk ofcorrosion. To protect the equipment, it is common to provide a galvanicanode (also known as a “sacrificial anode”) on the equipment, thusproviding cathodic protection (CP) for the equipment. Such anodescomprise sacrificial metals which have a lower electrochemical potentialthan the metal of which the equipment is made, such that when deployed,the sacrificial material of the anode is corroded more readily than themetal of the equipment. Currently, the most common metal materials usedas the sacrificial material for galvanic anodes include alloys ofaluminium, magnesium and zinc, with aluminium and zinc being favouredfor subsea use.

Although all sea-deployed equipment is at risk of such corrosion, thereare particular problems associated with hydrocarbon fluid extractionfacilities deployed on the sea bed, for example well trees. These arerelatively large, very expensive structures, which are often required tobe in place for over twenty years, and there may be major safety andenvironmental issues if corrosion occurs. Typically, well trees areprovided with U-shaped aluminium galvanic anodes, with the “legs” of theU being welded to the tree to ensure good electrical continuity. Atypical such anode is schematically shown in FIG. 1. As shown, the anodecomprises a block of aluminium 1, with a metallic leg 2 extending fromeach end. In use, the free ends of legs 2 are welded onto the well tree.Current practice is to weld the anodes to the tree frame (or otherstructure, manifold, template, flowbase etc) at the manufacturing stage.This gives good electrical continuity for the CP system to workefficiently.

The number of anodes/quantity of aluminium used is a function of theintended field life of the tree. For trees with a long life spans, manyanodes are required. For example, if calculations show that eight anodeswill be required for a field life of twenty years, then all eight anodeswill be welded to the tree frame (or other structure) in the workshop.This approach makes the tree densely populated, and fitting so manyanodes in may be difficult, and can result in anodes being closer tovulnerable areas than desired. In addition, placement of the anodes isimportant. Anode positions need to be considered carefully to bothminimize the dangers of hydrogen embrittlement and to optimize cathodicprotection.

It is an aim of the present invention to overcome these problems, andenable effective cathodic protection of underwater equipment, such aswell trees, for the duration of the equipment's operational life.

BRIEF SUMMARY OF THE INVENTION

In one embodiment of the present invention, an anode for cathodicprotection of underwater equipment is provided. The anode comprises: asupport body; sacrificial material retained by the support body; and anattachment mechanism for releasably attaching the anode to theunderwater equipment.

In another embodiment of the present invention, an anode comprising abuoyancy device configured to enable the density of the anode to beselected is provided.

In a further embodiment of the present invention, a method for providingcorrosion protection of underwater equipment is provided. The methodcomprises providing an anode for cathodic protection of the underwaterequipment, wherein the anode comprises: a support body, sacrificialmaterial retained by the support body, and an attachment mechanism forreleasably attaching the anode to the underwater equipment; andattaching the anode to the underwater equipment.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 schematically shows a known aluminium galvanic anode for use at asubsea well tree; and

FIG. 2 schematically shows a cross-sectional view of an anode inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following discussion and claims, the term “anode” is used todenote an item which includes sacrificial material, and not thesacrificial material itself

An embodiment of the invention is schematically shown in FIG. 2. Here,an anode 3 in accordance with an embodiment of the invention is shownjust prior to deployment at a well tree 4. The anode 3 is designed formanipulation by an ROV, however for clarity the ROV is not shown. Anode3 comprises an elongate support body 10, which supports and retains amass of sacrificial material 5. As shown, the sacrificial material 5 iscircumferentially moulded around the support body 10. The material 5may, for example, comprise a material selected from the group consistingof aluminium, zinc and magnesium, although other materials may be used.

One end of the support body 10 comprises a member formed as a taperedprojection 6. This is shaped to enable frictional engagement with thewell tree 4, this frictional engagement causing the anode 3 in use to beretained at the tree. More particularly, the projection 6 is tapered soas to be inserted (“stabbed”) and retained within a substantiallycorrespondingly shaped receptacle 7 provided at the well tree 4. Thetaper helps to ensure that electrical continuity is maintained betweenthe anode 6 and tree 4.

Receptacle 7 is specifically adapted for retaining anodes 3, which mustbe located at the tree prior to anode deployment, e.g. duringmanufacture. Receptacles 7 are placed at suitable locations on the treeframe that allow relatively simple deployment by ROV. Generally,receptacles would be positioned at the exterior of the tree 4, but inother embodiments receptacles could be placed in the body of the tree 4,such that, for example, anode 3 may be inserted into the receptacle inthe body of the tree from above (e.g. through a hole in the roof) orbelow (e.g. using horizontal tracks).

Friction is needed to force the surfaces of the projection 6 andreceptacle 7 to merge and ensure good electrical contact. Oncefrictionally-engaged, a locking mechanism (not shown) such as a holdinglatch acts to retain the anode 3 within the receptacle. This lockingmechanism may also be actuable by ROV. The latch may have variousdesigns, as would be appreciable to those skilled in the art, such as ascrew thread arrangement, or a lock bar/pin, latch, which is relativelysimple and relatively immune to crustacean damage.

As mentioned above, the anode 3 is designed for manipulation by an ROV.To this end, the anode comprises an ROV-friendly grab handle 8. Asshown, the handle 8 is mounted on the support body 10, at the distal endto the projection 6.

Anode 3 also includes buoyancy means for enabling the density of theanode to be selected, in this case comprising an air-filled cavity 9located within the support body 10. In effect therefore, the supportbody 10 could be considered to comprise a sealed, hollow pipe, ofsufficient strength to withstand the ambient pressure at theinstallation location. The dimensions and/or filling material of thecavity may be selected to make the anode 3 substantially buoyancyneutral. This provides various advantages, particularly that theoperation to install/replace the anode 3 would be much simpler and morecost effective. Below about one kilometre depth (i.e. the depth of thepycnocline), the density of water does not vary greatly with increasingdepth, however selection of the appropriate anode buoyancy maybedependent on the depth of installation. As an alternative, the anode maybe made slightly more dense than the sea water at the installation, suchthat in the event of accidental release, the anode would sink to the seafloor to facilitate recovery.

A suitable installation procedure may be as follows. As many anodes 3 asrequired are loaded onto an ROV launch frame, such launch frames beingknown in the art. The launch frame is picked up by an ROV and taken tothe required installation location. Individual anodes 3 are placed inrespective well tree receptacles 7 by the ROV, to create a friction fitbetween projection 6 and receptacle 7. The positive locking mechanism isengaged to more securely retain the anode 3 in the receptacle 7.Following initial installation, old, spent, anodes would be removed byROV before a new anode may be inserted. This would require disengagementof the positive locking mechanism, by the ROV. The anodes may beincluded in a regular inspection process using ROV (or diver) andcamera. Thus, visual inspection would determine when replacement wouldbe necessary.

The above-described embodiment is exemplary only, and otherpossibilities and alternatives within the scope of the invention will beapparent to those skilled in the art. For example for someinstallations, the buoyancy cavity 9 may be filled with materials otherthan air. For example, the cavity 9 may be filled with a solid material,so that the support body is more capable of withstanding high ambientpressure without undue deformation. Alternatively, the buoyancy meansmay comprise buoyancy tanks attached to the support body. The anodes mayoptionally be fitted with simple current/voltage monitoring means todetect when CP protection lowers to unacceptable levels, indicating thatreplacement of the anode is required. In this case the current/voltagemonitoring means could be connected to a condition monitoring system ofthe well.

Embodiments of the present invention provide various advantages over theprior art, including, but not limited to the following. Each anode maytake up less space on a tree than a conventional anode. Instead ofrequiring many anodes on a tree, relatively few need be used, thesebeing replaced as required. Consequently, there is more surface spaceavailable on the tree for other purposes. Since anodes are replaced asrequired, there is the potential for trees to have very long lives. Nowelding is required for embodiments of the present invention. Sinceanodes are modular, the overall weight of the tree is reduced. There isno need to run each anode from the surface to the tree. Since use ofreplaceable anodes means that a stock of anodes may be run to the seabed by ROV deployment, the ROV may then pick up each anode in turn toplace it in position as required. This simplifies the activity by havingonly one deployment trip that could cover all equipment, e.g. trees andmanifolds for example, in the local area; and all operations may becarried out by ROV with no surface operations, i.e. replacement can becarried out in bad weather. Replacement may be performed for exampleduring a tree inspection visit.

1. An anode for cathodic protection of underwater equipment, the anodecomprising: a support body; sacrificial material retained by the supportbody; and an attachment mechanism for releasably attaching the anode tothe underwater equipment.
 2. The anode of claim 1, wherein theattachment mechanism comprises a member for frictional engagement withthe underwater equipment, wherein the frictional engagement causes theanode to be retained in use by the underwater equipment.
 3. The anode ofclaim 2, wherein the member comprises a tapered projection from thesupport body for insertion and retention within a receptacle located atthe underwater equipment.
 4. The anode of claim 1, wherein theattachment mechanism comprises a releasable positive locking mechanism.5. The anode of claim 1, further comprising a handle designed formanipulation by a remotely operated vehicle.
 6. The anode of claim 5,wherein the handle is mounted on the support body.
 7. The anode of claim1, wherein the attachment mechanism comprises a member for frictionalengagement with the underwater equipment, wherein the frictionalengagement causes the anode to be retained in use by the underwaterequipment and the anode comprises a handle designed for manipulation bya remotely operated vehicle.
 8. The anode of claim 7, wherein the handleis mounted on the support body.
 9. The anode of claim 1, furthercomprising a buoyancy device configured to enable the density of theanode to be selected.
 10. The anode of claim 9, wherein the buoyancydevice comprises a cavity located within the support body.
 11. The anodeof claim 10, wherein the cavity is air-filled.
 12. An anode comprising abuoyancy device configured to enable the density of the anode to beselected.
 13. The anode of claim 12, wherein the buoyancy devicecomprises a cavity located within a support body of the anode.
 14. Theanode of claim 13, wherein the cavity is air-filled.
 15. The anode ofclaim 1, wherein the sacrificial material is selected from the groupconsisting of aluminium, zinc and magnesium.
 16. The anode of claim 1,wherein the underwater equipment comprises a subsea well tree.
 17. Amethod for providing corrosion protection of underwater equipment, themethod comprising: providing an anode for cathodic protection of theunderwater equipment, the anode comprising: a support body, sacrificialmaterial retained by the support body, and an attachment mechanism forreleasably attaching the anode to the underwater equipment; andattaching the anode to the underwater equipment.
 18. The method of claim17, wherein attaching the anode to the underwater equipment comprisesmanipulating the anode by a remotely operated vehicle.
 19. The method ofclaim 17, wherein the anode comprises a buoyancy device configured toenable the density of the anode to be selected.
 20. The method of claim17, wherein the underwater equipment comprises a subsea well tree.